1. Field of the Invention
The present invention relates to a charged particle beam scanning method and a charged particle beam apparatus. More particularly, it relates to a method and an apparatus for rotating the scan direction of a charged particle beam.
2. Description of the Related Art
At fabrication and inspection steps of a functional-device product fabricated by micromachining of its surface, such as a semiconductor device or thin-film magnetic head, a scanning electron microscope is widely used for performing measurement and external-appearance inspection of the micromachined pattern width.
The scanning electron microscope, which is one of charged particle beam apparatuses, is the following apparatus: An electron beam is emitted from an electron source, then being narrowly converged by a condenser lens and an objective lens. Next, the narrowly-converged beam is scanned on a sample in a one-dimensional or two-dimensional manner by using a deflector. Moreover, a secondary signal (i.e., secondary electrons, reflected electrons, and electromagnetic waves), which is generated from the sample by the electron-beam irradiation, is detected using a detector that utilizes photoelectric effect or the like. Finally, the sample image is formed by converting and processing the detected signal into a visual signal such as a luminance signal which is synchronized with the scanning with the electron beam.
In the scanning electron microscope, it turns out that the sample surface to be observed is irradiated with an electron beam having a-few-hundred-of-eV or more attainment energy.
In recent years, further microminiaturization has been implemented in the micromachining of the surface of a semiconductor device. In accompaniment with this trend, for example, it has been becoming more and more common to use, as photosensitive material for the photolithography, a photoresist which reacts with argon-fluoride (ArF) excimer laser light (this photoresist, hereinafter, will be referred to as “ArF resist”).
Since the ArF laser light is significantly short in wavelength, i.e., 160 nm, the ArF resist is suitable for the exposure of a more microscopic circuit pattern. The ArF resist, however, is exceedingly vulnerable to the electron-beam irradiation. As a result, there has been known the following phenomenon: When the formed pattern is observed by using the scanning electron microscope, the scanning with the focused electron beam causes a condensation reaction to occur in the base material such as acrylic resin. As a result, volume of the ArF resist decreases (this volume decrease, hereinafter, will be referred to as “shrink”). This shrink results in a change in configuration of the circuit pattern.
In a semiconductor device, the implementation of its design performance requires that configuration and dimension of the circuit pattern be strictly managed. For this purpose, a critical dimension scanning electron microscope, which is capable of measuring a microscopic dimension, is used at the inspection step. Nevertheless, when, at the observation and measurement steps, the electron-beam irradiation for the measurement results in a change in the circuit-pattern configuration, it becomes impossible to obtain a desired design value as the circuit-pattern dimension. Accordingly, there exists a problem of causing occurrence of characteristics degradation or crash in the semiconductor device.
Moreover, since the line width changes, there exists the following problem: Even if the same dimension is measured, the resultant measurement value varies every time the measurement is made. This variation prevents the measurement accuracy from being enhanced.
In addressing this problem, in US 2004/0051040 A1, the proposal has been made concerning a method for lowering irradiation density of the focused electron beam thereby to suppress the shrink of the ArF resist. In an ordinary scanning electron microscope, magnification in the horizontal direction and magnification in the vertical direction are made to coincide with each other. In contrast thereto, the above-described irradiation-density lowering is implemented by setting the magnification in the vertical direction lower as compared with the magnification in the horizontal direction, and performing the scanning in such a manner that the scan-line interval is enlarged.
Also, in JP-A-10-3876, the disclosure has been made concerning the technology about the so-called raster rotation, which is a scanning method where the scan direction of an electron beam is rotated.